1. Field of the Invention
The present invention relates generally to fuel cell systems, and more particularly to new and improved gas diffusion media for use in Proton Exchange Membrane (PEM) fuel cell systems.
2. Discussion of the Related Art
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells as well as in other fuel cell types, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in commonly-assigned U.S. Pat. Nos. 5,272,017 and 5,316,871, the entire specifications of which are incorporated herein by reference.
PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. PEM.fuel cells usually employ bipolar plates with channels on either side for the distribution of reactants over the electrode (i.e., anode and cathode) catalyst layer surfaces. Gas diffusion media (also known as gas diffusers or gas-diffusion backings) are provided between each face of the catalyst-coated proton exchange membrane and the bipolar plates. The region between reactant channels consist of lands, also known as ribs. Accordingly, in this type of design, roughly half of the electrode area is adjacent to the ribs and half is adjacent to the lands. The role of the gas diffusion media is to transition the anode and cathode gases from the channel-rib structure of the flow field to the active area of the electrode with minimal voltage loss. Although all of the current passes through the lands, effective diffusion media promote a uniform current distribution at the adjacent catalyst layers.
Examples of technology related to PEM and other related types of fuel cell systems can be found with reference to commonly-assigned U.S. Pat. No. 3,985,578 to Witherspoon et al.; U.S. Pat. No. 5,624,769 to Li et al.; U.S. Pat. No. 5,776,624 to Neutzler; U.S. Pat. No. 6,277,513 to Swathirajan et al.; U.S. Pat. No. 6,350,539 to Wood, lll et al.; U.S. Pat. No. 6,372,376 to Fronk et al.; U.S. Pat. No. 6,376,111 to Mathias et al.; U.S. Pat. No. 6,521,381 to Vyas et al.; U.S. Pat. No. 6,524,736 to Sompalli et al.; U.S. Pat. No. 6,566,004 to Fly et al.; U.S. Pat. No. 6,663,994 to Fly et al.; U.S. Pat. No. 6,793,544 to Brady et al.; U.S. Pat. No. 6,794,068 to Rapaport et al.; U.S. Pat. No. 6,811,918 to Blunk et al.; U.S. Pat. No. 6,824,909 to Mathias et al.; U.S. Patent Application Publication Nos. 2004/0009384 to Mathias et al.; 2004/0096709 to Darling et al.; 2004/0137311 to Mathias et al.; 2005/0026012 to O'Hara; 2005/0026018 to O'Hara et al.; 2005/0026523 to O'Hara et al.; 2005/0042500 to Mathias et al.; 2005/0084742 to Angelopoulos et al.; 2005/0100774 to Abd Elhamid et al.; and 2005/0112449 to Mathias et al., the entire specifications of all of which are expressly incorporated herein by reference.
The gas diffusion media provide reactant gas access from the flow field channel to the catalyst layers, provide a passage for removal of product water from the catalyst layer area to the flow field channels, provide electronic conductivity from the catalyst layers to the bipolar plates, provide for efficient heat removal from the MEA to the bipolar plates where coolant channels are located and provide mechanical support to the MEA in case of large reactant pressure difference between the anode and cathode gas channels. The above functions impose electrical and thermal conductivity requirements on the diffusion media including both the bulk properties and the interfacial conductivities with the bipolar plates and the catalyst layers. Due to the channel-rib structure of the bipolar plates, the gas diffusion media also allow gas access laterally from the channels to the catalyst area adjacent to the lands to allow for electrochemical reaction there. The gas diffusion media also promote water removal laterally from the catalyst area adjacent to the land out to the channel. The gas diffusion media also provide electronic conductivity laterally between the bipolar plate land and the catalyst layer adjacent to the channel, and maintains good contact with the catalyst layer for electrical and thermal-conductivity and must not compress into the channels resulting in blocked flow and high channel pressure drops.
State-of-the-art diffusion media in proton-exchange-membrane (PEM) fuel cells consist of carbon fiber mats, often called carbon fiber paper. These papers use precursor fibers made typically from polyacrylonitrile, cellulose, and other polymeric materials. The processing consists of forming the mat, adding a resin binder, curing the resin with the material (sometimes done under pressure and called molding), and progressively heating the material under inert atmosphere or vacuum to remove non-carbonaceous material. The final step in making the material is a high temperature heat treatment step that approaches or exceeds 1,600° C. reaching as high as 2,800° C. in some cases. This step is done in an inert gas (e.g., nitrogen or argon) or a vacuum environment, and the purpose is to remove non-carbonaceous material and, when the temperature is taken to approximately 2,000° C. or above, convert the carbon into graphite. This step can be done continuously or in batch furnaces using stacks of square sheets of carbon fiber paper, usually one meter square. Converting the carbon to graphite results in superior electrical conductivity that has typically been understood to be ideal for use in PEM fuel cells. Carbon fiber papers are also used as gas diffusion electrodes in phosphoric acid fuel cell (PAFC) applications. In that application, the material must be graphitized in order to have sufficient corrosion resistance to withstand the hot phosphoric acid electrolyte.
Summarizing, the manufacturing steps for a typical carbon fiber paper-based gas diffusion media typically include: (1) a carbon fiber paper manufacturing step; (2) an impregnation step using resin and fillers; (3) a resin curing step sometimes done with applied pressure; and (4) a carbonization/graphitization step. The carbon fiber paper manufacturing and impregnation steps are typically continuous, whereas the molding, carbonization and graphitization steps may be either batch or continuous. For example, one specific process includes the following steps: (1) a providing of polyacrylonitrile (PAN) fibers step; (2) a carbonization and chopping of the fibers step; (3) a paper making step, including adding 5-15% binder; (4) a resin impregnation step using phenolic resin; (5) a molding step; and (6) a carbonization/graphitization step.
The resin, typically phenolic, that is used to bond the structure together is conventionally applied during the separate impregnation step via a solution, with the solvent being driven off in high velocity ovens. However, all of these separate manufacturing and processing steps are time-consuming and costly.
Accordingly, there exists a need for new and improved gas diffusion media for PEM fuel cell systems.